Crystal oscillators or crystal oscillator circuits are well known in the art. One characteristic of crystal oscillators is their ability to provide extremely stable operation. As such, crystal oscillator circuits are best known for their extremely stable operation. Additionally, crystal oscillators advantageously provide low phase noise performance. Further, crystal oscillators are low cost, with high Q crystals of small size. A disadvantage, however, with respect to current crystal oscillators is their inability to provide wide band operation (i.e. they have a small pulling range).
The present invention relates to a type of crystal oscillator of which the frequency may be controlled by an electrical voltage, which enables the operating frequency of the oscillator to be freely selected within a given band of frequencies. The given band of frequencies is typically defined by the oscillator and/or oscillator component(s). This type of crystal oscillator is known as a voltage controlled crystal oscillator (VCXO).
The range of operating frequencies for a VCXO may be termed the band or tuning range of operating frequencies. A VCXO typically has a center frequency of operation with the upper (maximum) and lower (minimum) frequency of operation being the pulling range of the VCXO. A problem is that traditional voltage controlled crystal oscillators cannot be pulled by an amount greater than about 0.05% (e.g. +/−2.5 KHz pulling on a 10 MHz crystal).
What is therefore needed is a voltage controlled crystal oscillator that provides a pulling range that is greater than the prior art.